In general, an inverter connected to a commercial system for supplying AC power is called a system-interconnected inverter. The system-interconnected inverter supplies generated electric power to a connected AC load in cooperation with a commercial power supply or any other commercial system.
A system-interconnected inverter can also perform a backup operation as an electric generator when disconnected from a system in the event of a power outage or any other similar situation. When a capacitor-filtered diode bridge rectifier load (hereinafter simply referred to as a rectifier load) is connected, however, it is known that the waveform of the voltage outputted from the rectifier load suffers from harmonic distortion.
To reduce the harmonic distortion described above, a known conventional power supply apparatus switches between conduction/non-conduction of a rectifier device by comparing the voltage across a smoothing capacitor with the voltage which is outputted from rectifying means and on which a high-frequency AC voltage is superimposed, and conducts an input current from an AC power supply substantially all over one cycle of the AC voltage from the AC power supply so that there is no period during which the input current is not conducted (see, for example, Japanese Patent Application Laid-Open Publication No. H07-143758).
To reduce harmonic distortion and ripple voltage, another known power supply apparatus includes a charger that accumulates energy from a DC power supply, superimposes the accumulated energy on the output from the DC power supply, and discharges the resultant energy to each capacitor (see Japanese Patent Application Laid-Open Publication No. H08-308249, for example).
Each of the power supply apparatus disclosed in the H07-143758 publication and the H08-308249 publication, however, operates as a power supply apparatus alone but does not operate in connection with, for example, a commercial system, like a system-interconnected inverter.
As described above, when a system-interconnected inverter connected to a rectifier load performs backup operation, the output voltage waveform suffers from harmonic distortion. In general, an LC filter that forms the output stage of a system-interconnected inverter has a cutoff frequency fc as small as possible to the extent that the output frequency is not adversely affected. Harmonic components of the output from the system-interconnected inverter can thus be efficiently reduced.
Further, at the time of system interconnection, increasing the capacitance C of the LC filter increases a reactive current flowing through the capacitance C. To maintain a high power factor, it is necessary to cancel the reactive current flowing through the capacitance C. To this end, the capacitance C is desirably small.
In view of this fact, the LC filter is designed in such a way that the capacitance C has a smallest possible value and the inductance L has a large value at the time of system interconnection. When the capacitance C is small, it is difficult to follow instantaneous variation in load, which causes no problem when the system normally operates because any power shortage is compensated from the system power source.
At the time of the backup operation described above, the system-interconnected inverter operates as an electric generator, and the output current is not controllable but depends on the connected load because there is no reference voltage from the system power source at the time of backup operation.
The capacitance C of the LC filter is also desirably small for the same reason relating to system interconnection but requires an appropriate magnitude in accordance with the output from the electric generator and an intended load, since it is necessary to follow abrupt variation in load in order to operate the system-interconnected inverter as an electric generator.
FIG. 9 hereof shows the ranges of an LC filter constant required at the time of system interconnection and backup operation when the system-interconnected inverter operates as an electric generator alone. In FIG. 9, the character b denotes the range of the LC filter constant at the time of system interconnection, and the character c denotes the range of the LC filter constant at the time of backup operation. As shown in FIG. 9, the LC filter constant a (cutoff frequency) required at the time of system interconnection differs from that at the time of backup operation.
When a system-interconnected inverter operates at the time of backup operation, optimum inductance L and capacitance C cannot be used. It is therefore expected that the output voltage oscillates when the load abruptly varies and hence harmonic distortion of the output voltage worsens.
FIG. 10 shows measured harmonic distortion obtained when the LC filter constant is optimized not to worsen the harmonic distortion of the output current from the system-interconnected inverter described above interconnected with a system and the system-interconnected inverter is operated under that condition as an electric generator for backup operation. FIG. 10 hereof shows total harmonic distortion at an output voltage rating obtained when a resistance R load, an inductance L load, a capacitance C load, or a rectifier load is connected.
In FIG. 10, comparing the harmonic distortion [%] of the output voltage produced when the resistance R load, the inductance L load, or the capacitance C load is connected with the voltage produced when the rectifier load is connected indicates that the harmonic distortion produced when the rectifier load is connected is significantly large; i.e., 13.9%.
FIG. 11 hereof shows measured harmonic distortion components of various orders at the output voltage rating produced when the rectifier load is connected.
As indicated in FIG. 11, the harmonic distortion components for odd orders are large, and the harmonic distortion components for the third, fifth, eleventh, and thirteenth orders are particularly large.